The Money Flying Out the Back of Every Truck

Same Physics, Scaling Savings: How Bodybuilders Can Turn Drag into Differentiation

Here’s a number that should keep fleet owners awake at night: between R10,000 and R70,000 per vehicle, per year, disappearing into thin air. Literally.

Not from theft. Not from poor driving. Not from bad route planning. From the shape of the box on the back of the truck.

A note on the numbers: We’ve based these calculations on peer-reviewed research and adapted them for South African operating conditions. Your mileage — literally — may vary. Vehicle configuration, driving patterns, route profiles, and implementation quality all affect real-world results. These figures show what’s possible, not what’s guaranteed. Do your own math before committing capital.

A mini truck owner loses R10,000 annually to aerodynamic drag. Annoying, but survivable on tight margins. A 16-ton rigid fleet loses R25,000-R35,000 per truck — that’s a driver’s annual bonus vanishing into turbulence. An interlink operation hemorrhages R50,000-R70,000 per rig every year — enough to fund a full-time salary, evaporating behind the trailer.

Same physics. Same fixable problem areas. Same percentage of potential improvement.

The only difference is how many zeros follow the savings.

This isn’t a lecture on fluid dynamics. It’s a business case. If you build truck bodies or trailers, this is money you could be helping your customers keep — while charging them a premium for the privilege. If you buy trucks or manage fleets, this is money leaving your operation that your competitors might already be working to retain.

The question isn’t whether aerodynamic drag costs money. The physics settled that decades ago. The question is: who’s going to do something about it first?

Why Box Trucks Fight the Air (And Lose)

Every vehicle moving through air has to push that air out of the way. The energy required to do this pushing is called aerodynamic drag, and it translates directly into fuel consumption.

Streamlined shapes — like passenger cars, aircraft, or high-speed trains — slice through air efficiently. The air flows smoothly around curves, separates cleanly at the back, and the vehicle moves with minimal resistance.

Box trucks are the opposite of streamlined. They’re what engineers call “bluff bodies” — flat faces, sharp corners, rectangular profiles. Air doesn’t flow smoothly around a box. It slams into flat surfaces, tumbles around sharp edges, and creates chaotic turbulence that sucks the vehicle backward.

Think of it this way: Imagine walking through waist-deep water. Now imagine walking through waist-deep water while pushing a refrigerator in front of you. That’s roughly the difference between a streamlined car and a box truck fighting through air at highway speed.

The air doesn’t know or care that your cargo needs to be rectangular. It just knows that flat surfaces and sharp corners create pressure differences and turbulent zones that resist forward motion. And resistance costs fuel.

Where the Drag Actually Comes From

Here’s what most people — including many bodybuilders — don’t realise: the drag on a box truck isn’t spread evenly across the whole vehicle. It concentrates in specific zones, and a small number of areas account for the vast majority of the problem.

What you’re seeing: A side view of a typical box truck, colour-coded to show where air resistance concentrates. Red areas indicate high pressure (air pushing against the truck). Blue areas indicate low pressure (air creating suction that pulls the truck backward). Purple swirling patterns show turbulence — chaotic air motion that wastes energy.

The diagram reveals something crucial: 60-75% of total aerodynamic drag comes from just three zones. And all three exist outside the cargo space — meaning they can be addressed without sacrificing a single cubic centimetre of payload volume.

Let’s examine each zone and translate what it means for your fuel bill.

The Seven Drag Zones: Where Your Fuel Money Goes

Zone 5: The Rear Wake (25-35% of Total Drag)

• What’s happening: When air flows along the sides and roof of a box truck, it reaches the back and has nowhere to go. It separates from the vehicle and creates a large, low-pressure zone behind the truck — essentially a partial vacuum that sucks the vehicle backward.
• The visual explanation: Picture a boat moving through water. Behind the boat, you see churning, turbulent water filling in where the hull was. The same thing happens with air behind a truck, but you can’t see it. This churning wake creates suction. The bigger the flat rear face, the bigger the wake, the stronger the suction, the more fuel required to overcome it.
• Why fleet owners should care: On a standard box truck, the rear wake alone accounts for 25-35% of aerodynamic drag. For a 16-ton rigid truck burning R360,000 in fuel annually, that’s R27,000-R38,000 per year being sucked out of the back of the vehicle. For an interlink, it’s R50,000-R90,000 annually.
• Why bodybuilders ignore it: The rear doors need to open for loading. Everyone assumes you can’t do anything about a surface that has to swing open. This assumption is wrong — but convenient.

Zone 1: The Cab-to-Box Gap (15-25% of Total Drag)

• What’s happening: The space between the cab and the loadbox creates a trap for air. At highway speeds, air rushes into this gap and has nowhere to go cleanly. It tumbles, swirls, and creates intense turbulence that scrubs energy from the vehicle’s motion.
• The visual explanation: Imagine driving with your window open versus closed. That buffeting sensation you feel with the window cracked? That’s small-scale turbulence. Now imagine that happening in a gap large enough to stand in, with air moving at 80-120 km/h. The energy wasted in that turbulent churning comes directly from your fuel tank.
• Why fleet owners should care: The cab-box gap typically accounts for 15-25% of drag. On longer vehicles where this gap is larger, or on articulated combinations where the trailer nose sits well behind the cab, this zone can be even more significant. A rigid truck might lose R15,000-R25,000 annually to gap turbulence alone.
• Why bodybuilders ignore it: “That’s how trucks look.” The gap exists because cabs and bodies are built by different companies, mounted at different times, with different priorities. Nobody optimises the transition because nobody owns the transition.

Zone 2: The Box Front Face (10-15% of Total Drag)

• What’s happening: When air hits the flat front of the loadbox, it has to go somewhere — up, down, or around the sides. At each sharp 90-degree corner, the airflow separates violently, creating turbulent vortices that trail along the entire length of the vehicle.
• The visual explanation: Picture water hitting a brick wall versus water hitting a rounded river stone. The brick creates splash and chaos; the stone parts the water smoothly. Every sharp corner on a loadbox is a brick wall to the oncoming air, and every bit of splash and chaos costs fuel.
• Why fleet owners should care: Front face pressure and edge separation contribute 10-15% of drag. More importantly, the turbulence created here affects everything downstream — making the side surfaces and rear wake worse than they need to be.
• Why bodybuilders ignore it: Square corners are easier and cheaper to fabricate than radius corners. Welding a 90-degree joint takes less skill than forming a curved panel. The fabrication saving of a few hundred Rand costs customers thousands annually.

Supporting Zones (30-40% Combined)

• Roof leading edge (8-12%): Where air transitions from cab to box roof. Sharp edges cause separation; smooth transitions keep air attached.
• Side edge vortices (8-12%): Turbulent spirals created at the top and bottom corners of the loadbox that trail behind the vehicle, stealing energy.
• Underbody turbulence (10-15%): Air flowing under the vehicle encounters axles, fuel tanks, exhaust systems, and chassis members — each creating small drag penalties that add up.
• Wheel wells (5-10%): Rotating tyres fling air outward; exposed wheel wells create turbulent pockets that increase overall drag.

The Scaling Reality: Same Physics, Different Zeros

Here’s where the business case becomes compelling. The physics of drag doesn’t change based on vehicle size. A 25% improvement in aerodynamic efficiency saves 25% of the drag-related fuel cost regardless of whether it’s a 4-ton bakkie or a 56-ton interlink.

What changes is the baseline. And that’s where small percentages become large Rand values.

Mini Truck Profile (FAW 4.110 / Hino 300 Class)

The baseline reality:

• Typical drag coefficient (Cd): 0.65-0.75
• Annual distance: 50,000 km
• Fuel consumption: 12-15 L/100km
• Annual fuel spend: R108,000-R135,000
• Drag portion of fuel cost (~35%): R38,000-R47,000

What aerodynamic improvements can achieve:

• Optimised drag coefficient: 0.50-0.55
• Drag cost reduction: 20-25%
• Annual savings: R8,000-R12,000
• Five-year savings: R40,000-R60,000 per vehicle

For the fleet manager: A 10-vehicle mini truck fleet implementing basic aerodynamic improvements saves R80,000-R120,000 annually. That’s a part-time salary appearing from better airflow.

For the bodybuilder: An aerodynamic package costing R15,000-R20,000 premium saves the customer R40,000+ over five years. That’s an easy sale to anyone who can operate a calculator.

Rigid Truck Profile (MAN TGM 16-ton / UD Croner Class)

The baseline reality:

• Typical drag coefficient: 0.70-0.80
• Annual distance: 80,000 km
• Fuel consumption: 25-32 L/100km
• Annual fuel spend: R360,000-R460,000
• Drag portion of fuel cost (~30%): R108,000-R138,000

What aerodynamic improvements can achieve:

• Optimised drag coefficient: 0.55-0.60
• Drag cost reduction: 20-28%
• Annual savings: R25,000-R38,000
• Five-year savings: R125,000-R190,000 per vehicle

For the fleet manager: The medium-duty segment often operates on thin margins with high utilisation. A R30,000 saving per truck across a 20-vehicle fleet is R600,000 annually — enough to fund an additional vehicle’s operating costs.

For the bodybuilder: A R30,000-R45,000 aero package premium on a R350,000+ body build is under 15% price increase for a customer benefit of 3-4x the investment over vehicle lifetime.

Articulated Trailer Profile (Interlink / Superlink Class)

The baseline reality:

• Typical drag coefficient (combination): 0.75-0.90
• Annual distance: 120,000 km
• Fuel consumption: 38-48 L/100km
• Annual fuel spend: R820,000-R1,036,000
• Drag portion of fuel cost (~25%): R205,000-R259,000

What aerodynamic improvements can achieve:

• Optimised drag coefficient: 0.55-0.65
• Drag cost reduction: 25-30%
• Annual savings: R50,000-R78,000
• Five-year savings: R250,000-R390,000 per trailer set

For the fleet manager: Long-haul operations burn serious fuel, and aerodynamics matters more at sustained highway speeds. A 50-trailer operation implementing comprehensive aero packages saves R2.5-R3.9 million annually. This isn’t marginal — it’s transformational.

For the trailer manufacturer: A R70,000-R110,000 aero package on a R600,000+ trailer set represents 12-18% premium for customer returns of 3-5x over trailer lifetime. At these numbers, not offering an aero option is leaving money on the table.

The Solutions: What Actually Works

Understanding where drag comes from is useless without knowing how to fix it. Here’s the practical toolkit, ranked by effectiveness, cost, and implementation complexity.

Universal Solutions (All Vehicle Classes)

Radiused Front Corners

• What it is: Replacing the sharp 90-degree corners at the front of the loadbox with curved transitions (typically 75-150mm radius).
• Why it works: When air hits a rounded corner, it follows the curve and stays attached to the surface longer. When it hits a sharp corner, it separates immediately and creates turbulence. Keeping airflow attached reduces drag and improves everything downstream.
• What it looks like: From outside, the front corners of the loadbox appear slightly rounded rather than perfectly square. From inside, the cargo space remains fully rectangular — the radius exists only in the outer skin or is filled with foam/structure.

The numbers:

Vehicle Class	Implementation Cost	Drag Reduction	Annual Saving	Payback Period
Mini truck	R500-R1,500	3-5%	R1,500-R2,400	3-8 months
Rigid (16t)	R1,000-R2,500	3-5%	R4,500-R6,900	2-5 months
Articulated	R1,500-R4,000	3-5%	R7,500-R13,000	2-4 months

Why it’s not standard: Square corners are faster to fabricate. The few hundred Rand saved in production costs the customer thousands annually. This is the clearest example of bodybuilder optimisation versus customer optimisation.

Rear Vertical Strakes

• What it is: Vertical fins or panels (50-100mm depth) mounted at the rear corners of the loadbox, extending the corner line backward.
• Why it works: The sudden transition from vehicle side to open air creates violent flow separation. Strakes provide a controlled transition, guiding airflow inward and reducing the size and intensity of the rear wake. Think of them as “training wheels” for the air, helping it navigate the corner smoothly rather than tumbling chaotically.
• What it looks like: Small vertical fins visible at the rear corners when viewing the truck from behind. They don’t obstruct door opening and add only 50-100mm to vehicle length.

The numbers:

Vehicle Class	Implementation Cost	Drag Reduction	Annual Saving	Payback Period
Mini truck	R1,500-R3,000	5-8%	R2,500-R4,000	5-10 months
Rigid (16t)	R2,500-R5,000	5-8%	R7,500-R11,000	3-6 months
Articulated	R4,000-R8,000	5-8%	R15,000-R21,000	3-5 months

Why it’s not standard: Strakes add material cost, fabrication time, and slight vehicle length. Most bodybuilders have never been asked for them, so they’ve never tooled up to offer them efficiently.

Roof Trailing Edge Fairing

• What it is: An angled panel extending 150-300mm beyond the rear of the roof, angled downward at approximately 15 degrees.
• Why the 15-degree angle matters: This specific angle isn’t arbitrary — it’s the point where airflow stays attached to the surface longest before separating. Steeper angles cause immediate separation (defeating the purpose). Shallower angles don’t guide air far enough into the wake zone. The 15-degree sweet spot directs high-energy air from the roof downward into the low-pressure wake, partially filling the vacuum and reducing suction.
• What it looks like: A slight “lip” or extension at the back of the roof, angling downward. Similar in concept to a rear spoiler on a car, but designed for drag reduction rather than downforce.

The numbers:

Vehicle Class	Implementation Cost	Drag Reduction	Annual Saving	Payback Period
Mini truck	R2,500-R4,000	3-5%	R1,500-R2,500	12-20 months
Rigid (16t)	R4,000-R7,000	3-5%	R4,500-R7,000	8-14 months
Articulated	R6,000-R12,000	3-5%	R7,500-R13,000	6-12 months

Why it’s not standard: Adds height to overall vehicle (potential bridge/overhead clearance issues), requires additional fabrication steps, and hasn’t been demanded by customers who don’t know to ask for it.

Class-Specific Solutions

Mini Truck: Cab-Box Collar

• What it is: A flexible or rigid fairing that bridges the gap between cab and loadbox, preventing air from entering and churning in the gap.
• Why it works: Eliminates the turbulence zone entirely by preventing air entry. Air flows smoothly from cab to box without the energy-wasting tumble in the gap.
• Implementation cost: R2,000-R4,000 Drag reduction: 8-12% Annual saving: R4,000-R6,000 Payback: 4-8 months
• Why it’s compelling: Single biggest bang-for-buck improvement on mini trucks. Simple fabrication, clear visual differentiation, and sub-6-month payback makes this the obvious first addition to any “aero package” option.

Rigid Truck: Integrated Cab Fairing + Side Skirts

• What it is: A full fairing system that smooths the cab-to-box transition (top and sides) combined with panels closing the gap between loadbox floor and ground.
• Why it works: The larger gap on rigid trucks creates proportionally more turbulence. Full fairings eliminate this entirely. Side skirts prevent air from tumbling under the vehicle and disrupting the wake.
• Combined implementation cost: R14,000-R27,000 Combined drag reduction: 14-21% Annual saving: R15,000-R29,000 Payback: 10-14 months
• Why it’s compelling: At the 16-ton class, fuel costs are significant enough that fleet managers actively look for savings. An aero package at this level delivers returns visible on quarterly financial statements.

Articulated Trailer: Gap Fairing + Side Skirts + Boat Tail

• What it is: A comprehensive system addressing the three major drag zones: cab-trailer gap, underbody exposure, and rear wake.
  • Gap fairing closes the space between tractor cab and trailer nose, preventing turbulence in what is often a 1-meter+ gap.
  • Side skirts (on both trailers for interlinks) seal the underbody from ground-level airflow, preventing the chassis, axles, and suspension from creating drag.
  • Boat tail addresses the rear wake through angled panels that taper inward at the back of the trailer. The term comes from boat hull design — the tapered stern that allows water to close smoothly behind the vessel. On a trailer, folding boat tail panels angle inward at 15 degrees from vertical, guiding air into the wake zone and dramatically reducing the low-pressure suction effect.
• Why folding matters: Fixed boat tails would prevent reversing to loading docks. Folding designs collapse flat against the doors during loading operations and deploy automatically at highway speeds, providing full aerodynamic benefit without operational compromise.
• Combined implementation cost: R45,000-R75,000 Combined drag reduction: 19-28% Annual saving: R40,000-R72,000 Payback: 12-18 months
• Why it’s compelling: A superlink operation running 50 trailer sets with comprehensive aero packages saves R2-R3.6 million annually. At that scale, aerodynamic optimisation isn’t a nice-to-have — it’s a competitive necessity. Operators without it are subsidising their competitors’ fuel bills.

The Compounding Reality

The numbers above represent single-vehicle, single-year savings. But transport operations compound these effects:

• Fleet multiplication: A 20-truck rigid fleet implementing full aero packages saves R300,000-R580,000 annually. That’s not a rounding error — it’s operational transformation.
• Time multiplication: Trucks operate for 10-15 years. A R30,000 annual saving becomes R300,000-R450,000 over vehicle lifetime. Suddenly the R40,000 aero package premium looks like one of the best investments in the fleet.
• Fuel price multiplication: When these calculations were drafted, diesel sat at approximately R18/L. Every R1 increase in fuel price increases the value of aerodynamic savings proportionally. At R22/L, all savings increase by 22%. At R25/L — entirely possible within the next few years — savings increase by 39%.

The R10,000 annual saving on a mini truck at today’s prices becomes R14,000 at tomorrow’s prices. The R70,000 saving on an interlink becomes R97,000. Aerodynamic investments become more valuable with every fuel price increase, while the improvement itself is paid for once.

The Altitude Factor

One additional consideration for South African operations, particularly in Gauteng: altitude affects aerodynamic calculations.

Air density decreases at altitude. At Johannesburg’s 1,750m elevation, air is approximately 18% less dense than at sea level. This means:

• Less drag force (good news): Lower air density means less resistance at any given speed.
• But proportionally similar savings (neutral): While absolute drag forces are lower, the percentage improvement from aerodynamic modifications remains similar. A 25% improvement is still 25% at altitude.
• And higher fuel consumption (bad news): Engines work harder at altitude due to reduced oxygen, meaning vehicles burn more fuel per kilometre for all purposes, including overcoming (reduced but still significant) drag.

The net effect: Aerodynamic improvements in Gauteng deliver similar percentage savings to coastal operations, applied to higher baseline fuel consumption. The Rand savings per kilometre may actually be slightly higher for Highveld operations.

The Business Case for Bodybuilders and Trailer Manufacturers

Here’s the uncomfortable truth: the current approach to truck body fabrication optimises for the manufacturer’s costs, not the customer’s costs.

Square corners are cheaper to fabricate than radius corners. Flat panels are simpler than curved fairings. Basic builds require fewer jigs, less skilled labour, and simpler material handling than aerodynamic designs.

These savings might amount to R500-R5,000 per vehicle in reduced fabrication costs. They cost customers R10,000-R70,000 per year in additional fuel consumption.

This math only works when customers don’t know to ask for better. The moment fleet managers start comparing lifecycle costs rather than purchase prices, the calculation inverts entirely.

The opportunity for forward-thinking manufacturers:

1. Differentiation: In a market where everyone builds the same boxes, an “Aero Package” option stands out. Fleet procurement managers seeing identical quotes will notice the one offering quantified fuel savings.
2. Premium pricing: A R40,000 premium on a R300,000 body build is 13% more expensive. But it delivers R150,000+ in customer savings over vehicle lifetime. That’s not expensive — that’s value.
3. Relationship depth: Offering aerodynamic options positions you as a partner optimising customer outcomes, not a vendor selling commodities. That changes the conversation and the relationship.
4. First-mover advantage: The South African market hasn’t woken up to this yet. The first manufacturers tooling up for aerodynamic production will capture the fleet customers who calculate total cost of ownership. The rest will compete on price for the customers who don’t.

Why This Hasn’t Happened Yet

If the savings are so clear, why isn’t every truck in South Africa wearing aero fairings?

• Bodybuilders optimise for their costs: Fabrication efficiency, not customer operating costs, drives design decisions. This isn’t malicious — it’s rational response to market pressure. When customers choose on purchase price, suppliers compete on purchase price.
• Fleet buyers focus on capital, not operations: Procurement departments often operate separately from operations departments. The team negotiating vehicle prices may not bear responsibility for fuel costs. Different budget lines, different incentives.
• “That’s how we’ve always done it”: Inertia is powerful. Square boxes have worked for decades. Challenging assumptions requires effort, and effort has costs.
• Nobody asks for it: Customers don’t demand aerodynamic optimisation because they don’t know it’s available or quantifiable. Suppliers don’t offer it because customers don’t ask. The cycle perpetuates.
• No regulatory pressure: European markets increasingly mandate CO2 efficiency improvements that drive aerodynamic adoption. South Africa has no such requirements. Without regulatory push, market pull has to be sufficient — and so far, it hasn’t been.

This means the opportunity remains open for early movers. The manufacturers who develop aerodynamic capabilities now will be positioned when the market inevitably shifts toward lifecycle cost optimisation.

The Challenge

The physics is settled. The savings are quantifiable. The solutions exist.

What remains is execution.

• For bodybuilders and trailer manufacturers: The capability to build aerodynamic bodies exists within current skillsets. Radius corners, fairings, and strakes don’t require exotic materials or revolutionary processes. They require recognition that customer operating costs matter, and willingness to tool up for slightly more complex builds.
• For fleet owners and transport operators: The numbers in this article aren’t theoretical — they’re based on established aerodynamic principles applied to South African operating conditions. R10,000 to R70,000 per vehicle per year isn’t a marketing claim; it’s physics.
• For procurement managers: The next RFQ you issue could include aerodynamic specifications. Ask for drag coefficients. Ask for quantified fuel consumption estimates. Ask what manufacturers are doing about the rear wake, the cab gap, the front face. The suppliers who can answer those questions are the ones investing in your operating costs, not just their own margins.

Conclusion: The R10,000 to R70,000 Question

Every truck and trailer on South African roads is paying an aerodynamics tax. The flat faces, sharp corners, and turbulent gaps that make vehicles easier to build make them harder to run.

For mini trucks, that tax runs R10,000-R12,000 per year in unnecessary fuel consumption.

For 16-ton rigids, R25,000-R38,000 annually.

For interlinks and superlinks, R50,000-R78,000 every year, trailing away in the turbulent wake.

This money is leaving your operation regardless. The only question is whether you’re going to start keeping some of it.

Same physics. Same fixable zones. Same solutions.

Different zeros on the savings — determined entirely by vehicle size.

• Which South African bodybuilder will be first to offer an aerodynamically optimised option?
• Which trailer manufacturer will market fuel savings as a product feature?
• Which fleet operator will include aerodynamic specifications in their next vehicle tender?

The R10,000 to R70,000 is leaving your customers’ accounts every year. The physics demands it. The question is whether someone helps them keep it — and takes their cut for the service.

References & Research Sources

Primary Research Sources

The claims and figures in this article are supported by peer-reviewed research, government studies, and industry data from the following sources:

Government & Institutional Research

1. Aerodynamic Drag Reduction Technologies Testing of Heavy-Duty Vocational Vehicles and a Dry Van Trailer — National Renewable Energy Laboratory (NREL), 2016
   • Coastdown testing methodology for quantifying aerodynamic improvements
   • Real-world fuel savings simulations across vocational drive cycles
   • Box truck aerodynamic device testing (chassis skirts, front fairings, wheel covers)
2. Aerodynamic Drag Reduction of Class 8 Heavy Vehicles — Lawrence Livermore National Laboratory / US DOE, 2012
   • Comprehensive review of drag reduction devices for tractor-trailers
   • Gap fairings: average 3,000 L fuel savings per 200 million metres driven
   • Trailer skirts: average 5,000 L fuel savings per 200 million metres driven
   • Boat tails: average 4,000 L fuel savings per 200 million metres driven
   • Combined devices: approximately 12,000 L savings per 200 million metres driven
3. Reducing Aerodynamic Drag and Rolling Resistance — International Council on Clean Transportation (ICCT), 2012
   • Individual aerodynamic aids can reduce fuel use by 1-15%
   • Combined device packages can reduce fuel use by 25% or more
   • Aerodynamic benefits are speed-sensitive (greater savings at higher speeds)
4. Review of Aerodynamic Drag Reduction Devices for Heavy Trucks and Buses — Transport Canada / National Research Council Canada, 2012
   • Friction drag contributes only 10% or less of total heavy vehicle drag
   • Pressure drag dominates due to large perpendicular surfaces and blunt rear ends
   • Gap region, underbody, and base wake identified as primary drag sources
5. Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles — National Academies Press
   • Energy balance breakdowns for Class 8 tractor-trailers and medium-duty trucks
   • Post-SmartWay design evaluation by Truck Manufacturers Association and DOE
   • Gap fillers, side skirts, underbody treatments, and trailer features evaluated
6. A Summary of the Experimental Study of a Generic Tractor-Trailer — NASA Technical Reports Server
   • Tractor-trailer gap effects on drag coefficient
   • Trailer drag increases by factor of 3 as gap width increases
   • Wind-averaged drag measurements across yaw angles

EPA SmartWay Program

7. Learn About SmartWay Verified Aerodynamic Devices — US Environmental Protection Agency
   • Gap reducers, skirts, and tails can be installed individually or in combination
   • Multiple test methods: track testing, coastdown, CFD simulation
   • Verified devices achieve minimum 4-5% fuel savings
8. SmartWay Designated Tractors and Trailers — US EPA
   • SmartWay designated combinations can reduce fuel use by up to 20%
   • Annual savings of 2,000-4,000 gallons (7,500-15,000 litres) diesel per year
   • Aerodynamic devices achieving minimum 5% fuel savings qualify trailers

Peer-Reviewed Academic Research

9. Fuel Savings on a Heavy Vehicle via Aerodynamic Drag Reduction — Transportation Research Part D, 2010
   • Fuel reductions from individual devices range from <1% to almost 9%
   • Long-haul routes save approximately twice as much fuel as urban distribution
   • 22% of UK road vehicle energy consumed by heavy vehicles (only 1.3% of fleet)
10. Heavy Truck Drag Reduction Obtained from Devices Installed on the Trailer — SAE International Journal of Commercial Vehicles, 2015
    • Front-rear trailer devices reduce wake size and overall vehicle drag
    • Boat tail configurations achieve 3.5% drag reduction
    • Combined drag reductions validated through CFD and wind tunnel testing
11. Understanding Practical Limits to Heavy Truck Drag Reduction — SAE International, 2009
    • Maximum achievable wind-averaged drag reduction: approximately 31%
    • Testing included side skirts, full gap seal, and tapered rear panels
    • Old Dominion University / Langley Full Scale Wind Tunnel testing
12. Substantial Drag Reduction of a Tractor-Trailer Vehicle Using Gap Fairings — Journal of Wind Engineering and Industrial Aerodynamics, 2017
    • Gap fairings can reduce tractor-trailer drag by approximately 7% at highway speeds
    • PIV measurements confirm reduced velocity and turbulence in gap region
    • Quantitative drag-reduction mechanism analysis
13. Assessment of Drag Reduction Devices Mounted on a Simplified Tractor-Trailer Truck Model — University of Derby, 2021
    • Roof deflector + side extenders + vortex trap devices: 24% drag reduction
    • Primary drag reduction mechanism: reduced pressure on trailer front face
    • Gap turbulence reduction contributes less than front face pressure reduction
14. An Investigation into the Wake Structure of Square Back Vehicles — SAE International, 2011
    • Rear wake contributes 30% or more of total aerodynamic drag on square-back vehicles
    • Low base pressure in wake region is primary drag mechanism
    • Small passive modifications can produce large changes in drag and lift
15. Development of Aerodynamic Drag Reduction around Rear Wheel — SAE International, 2021
    • Rear of vehicle accounts for approximately 40% of overall aerodynamic drag
    • Low base pressure in wake region is the primary mechanism
    • Underfloor flow management can reduce wake size

Industry & Historical Research

16. DOE’s Effort to Reduce Truck Aerodynamic Drag — Joint Experiments and Computations Lead to Smart Design — US Department of Energy, 2004
    • Tractor-mounted aero devices provide drag reductions of 0.15-0.25 Cd
    • Fuel savings of 3,000-5,000 US gallons per year per truck achievable
    • Industry adopted tractor modifications as early as 1980s
17. Aerodynamics Research Revolutionizes Truck Design — NASA Spinoff, 2008
    • Closing cab-trailer gap: 20-25% fuel consumption reduction
    • Boat tail addition: approximately 15% drag reduction
    • Annual savings potential: up to 6,829 gallons (25,840 litres) per truck
18. Study: Improvements in Large Truck Aerodynamics Could Save US Nearly One Billion Gallons of Fuel Annually — Green Car Congress, 2006
    • Combined aerodynamic improvements: up to 23% drag reduction
    • For every 2% reduction in aerodynamic drag: 1% improvement in fuel efficiency
    • Four TMA members (75% of US Class 8 market) collaborated with DOE

Key Research Findings Summary

Finding	Source	Value
Rear wake contribution to total drag	SAE 2011-37-0015, SAE 2021-01-0962	30-40%
Friction drag contribution	Transport Canada/NRC	<10%
Gap fairing fuel savings	LLNL/DOE 2012	~7% at highway speeds
Side skirt fuel savings	NREL, ICCT	4-7%
Boat tail fuel savings	NREL, NASA	6-15%
Combined aero package potential	EPA SmartWay, ICCT	20-31%
Maximum practical drag reduction	SAE 2009-01-2890	~31%
SmartWay designated vehicle savings	US EPA	Up to 20% (2,000-4,000 gal/year)

Methodology Notes

Drag zone percentages cited in this article represent ranges compiled from multiple wind tunnel studies, CFD analyses, and on-road testing programs. Actual contributions vary based on:

• Vehicle geometry and configuration
• Operating speed and conditions
• Yaw angle (crosswind effects)
• Gap width and trailer length

Fuel savings calculations are based on:

• South African fuel prices (R18-R22/L diesel as baseline)
• Typical annual distances for each vehicle class
• Aerodynamic drag portion of total fuel consumption at highway speeds
• Conservative improvement percentages from validated research

South African context adjustments:

• Altitude effects on air density (Gauteng: 1,750m elevation)
• Local operating speeds (80-120 km/h highway)
• Vehicle configurations common to SA market

Additional Reading

• The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains — Springer Lecture Notes in Applied and Computational Mechanics (conference proceedings)
• SAE J1252: Wind Tunnel Test Procedure for Trucks and Buses — Industry standard test methodology
• ASHRAE Handbook: Fundamentals — Air density and atmospheric pressure calculations

Related Reading

• The Condenser Aerodynamics Paradox — How larger cooling systems can reduce drag
• High-Altitude Refrigeration Challenges — Why Gauteng operations need different specifications